The subject matter involved herein is broadly classified in class 156, subclass 210. Representative prior art is exemplified by U.S. Pat. No. 3,607,523 and the references cited in said co-pending application Ser. No. 614,050.
For many years, the platens of a double facer were heated by steam. Steam heat has certain disadvantages such as structural considerations due to the high pressure of steam when steam is at the temperature used in a double facer machine. Each steam heated platen is a potential source of danger when in operation due to the high pressure and the compressed state of the steam. Further, there is a limit to the practical upper temperature range for steam when it is used as a heat transfer medium, so that even if the temperature were raised to the superheated range, insufficient heat would be transferred to warrant a corresponding increase in the operating speed of the single facer.
When a relatively cool web of paperboard is placed in contact with the surface of a thin, elongated platen, such as the platen in said copending patent application, the temperature of the upper plate is reduced. As the upper plate is cooled by transfer of heat to the web, its length is reduced thereby causing the ends of the platen to curve upward. If the center of the platen is below the elevation of the ends, the outer areas of the corrugated paperboard will be compressed by the weight of the conventional overlying belt and the ballast rollers. In like manner, the central area of the paperboard will lack contact with the platen and wet board will result.
Accordingly, there has been great need for a double facer platen which will maintain a substantially flat upper surface irrespective of the heat load imposed by the paperboard web. This need exists in both steam heated cast iron chests and fabricated steel platens. The higher temperatures prevailing in the liquid heated double facer, however, have made the problem more acute. This invention recognizes the source of the problem and presents a variety of solutions.